The present invention relates to a substrate rotating apparatus for rotating a substrate to be processed, e.g. a semiconductor substrate, in a processing chamber. More particularly, the present invention relates to a substrate rotating apparatus for rotating a substrate in a processing chamber of a system in which a substrate is processed in a special atmosphere, e.g. in a CVD system for forming a thin film on a substrate, an etcher system for thinly removing the surface of a substrate, or a rapid thermal annealing (RTA) system.
In a system wherein a substrate is processed in a special atmosphere, e.g. in a cvb system or an etcher system, as shown in FIG. 1, a substrate 101 to be processed is mounted on a substrate holder or susceptor 102 and rotated in a processing chamber 103 by rotating the substrate holder or susceptor 102. This kind of system, however, involves some problems, e.g. generation of dust or release of gas from a rotating mechanism for rotating the substrate holder or susceptor 102, and a problem in terms of the lifetime of bearings. In view of these problems, there has been proposed a system in which magnetic bearings (radial bearings 105 and 106 and an axial bearing 107) are used as bearings for rotatably supporting a rotor 104.
In the above-described system wherein a substrate is processed in a special atmosphere, e.g. in a CVD system or an etcher system for thinly removing the surface of a substrate, a corrosive gas is often used in the processing chamber 103. Therefore, the gas contact portion of a stator-side constituent member of each magnetic bearing is covered with a cylindrical pressure resistive partition (can). However, as the diameter of the substrate 101 increases, the diameter of the rotor 104 of the rotating mechanism also increases. Consequently, it is necessary to increase the strength (rigidity) of the pressure resistive partition.
For example, when an external pressure of 1 kgfxc2x7cm2 is applied to a stainless steel cylindrical partition having an inner diameter of 400 to 500 mm, the wall thickness to withstand a buckling stress of 1 kgfxc2x7cm2 is about 3 to 5 mm. When the wall thickness is about 0.3 mm, i.e. about one tenth of the above-mentioned value, buckling stress is reached when the pressure difference is 1 Torr. Accordingly, as the diameter of the rotor 104 increases, the wall thickness of the partition also needs to be increased. Increasing the wall thickness of the partition causes an increase in the gaps between stator-side electromagnets 105a, 106a, 107a and 107b and rotor-side targets of the radial magnetic bearings 105 and 106 and the axial magnetic bearing 107, resulting in a reduction in control magnetic force. To obtain satisfactorily large control magnetic force, therefore, it is necessary to increase the number of windings of each electromagnet or to intensify the exciting current. This causes the magnetic bearings to become undesirably large in size. In FIG. 1: reference numeral 108 denotes a motor stator for applying rotational force to the rotor 104; 109 denotes a quartz window; 110 denotes a lamp heater unit for heating the inside of the processing chamber 103; and 111 and 112 denote load-unload gates for loading and unloading the substrate 101 into and out of the chamber 103.
Moreover, as the diameter of the rotor 104 increases, the diameters of the electromagnets 107a and 107b of the axial magnetic bearing 107 also increase. Accordingly, unstable torque (unbalanced torque) produced by the electromagnets 107a and 107b becomes so large that it cannot be ignored. If two radial magnetic bearings 105 and 106 are provided axially spaced (for 4 axes, two for each bearing) for the purpose of compensating for the unbalanced torque, the axial length increases, and hence the space efficiency decreases. Regarding the rotor characteristics, the design is dangerous because the inertia moment ratio of the rotor is close to 1.
In view of the above-described circumstances, an object of the present invention is to provide a substrate rotating apparatus which is capable of avoiding an increase in size of the diameter of the electromagnet of the axial magnetic bearing and capable of reducing unstable force such as unstable torque produced by the electromagnets, even if the diameter of the rotor is increased.
Another object of the present invention is to provide a substrate rotating apparatus which is capable of avoiding an increase in magnetic reluctance between the rotor-side target and the stator-side magnet yoke even if the partition provided therebetween is formed from thick-walled material and whereby a large control magnetic force can be obtained without enlarging the size of the magnetic bearings or without increasing the number of windings thereof.
To attain the above-described object, the present invention provides a substrate rotating apparatus including a rotor for rotating a substrate mounted thereon. The rotor has a horizontal disk provided thereon and is supported by magnetic bearings. The substrate rotating apparatus further includes a motor for applying rotational force to the rotor to rotate the substrate in a processing chamber. The magnetic bearings include an axial magnetic bearing and a radial magnetic bearing. The axial magnetic bearing is divided into three or more magnetic bearings, which are positioned so that imaginary lines connecting points where the divided axial magnetic bearings are disposed form an approximately regular triangle or polygon. Electromagnets constituting the axial magnetic bearings are disposed substantially above and below the horizontal disk of the rotor.
Because the axial magnetic bearing is divided into three or more magnetic bearings and these magnetic bearings are positioned so that imaginary lines connecting points where the divided axial magnetic bearings are disposed form an approximately regular triangle or polygon, even if the diameter of the rotor increases, the diameter of the electromagnet of each of the axial magnetic bearings will not increase. In addition, because position control in the axial direction is effected at the position of each axial magnetic bearing, motion about the radial axes is also stabilized. Accordingly, any unstable force such as unstable torque (unbalanced torque) produced by the electromagnets of the conventional axial magnetic bearing is minimized.
Preferably, the rotor is placed in a space communicating with the processing chamber, and stator-side constituent members of the magnetic bearings and a stator-side constituent member of the motor are placed in a space defined outside the space communicating with the processing chamber by a partition provided between the rotor and the stator-side constituent members. Further, a material electromagnetically equivalent to yokes of electromagnets as stator-side constituent members of the magnetic bearings is fitted in or inserted in portions of the partition where the yokes are located, and the partition constitutes a stator housing as a whole.
Instead, the ends of yokes of electromagnets as stator-side constituent members of the magnetic bearings may pierce through the partition so as to face the rotor directly.
If a material electromagnetically equivalent to the yokes of electromagnets as stator-side constituent members of the magnetic bearings is fitted in portions of the partition where the yokes are located, or the ends of yokes of electromagnets as stator-side constituent members of the magnetic bearings pierce through the partition so as to face the rotor directly, there is no increase in magnetic reluctance between the rotor-side target and the stator-side magnet yoke even if the partition, which constitutes the stator housing as a whole, is formed by using a thick-walled material. Accordingly, large control magnetic force can be obtained.
In the above-described substrate rotating apparatus, translational motion in two radial axes (X- and Y-directions) is preferably passively supported by a radial restoring force generated when the relative position between the yoke of an electromagnet as a stator-side constituent member of the axial magnetic bearing and the horizontal disk as a rotor-side constituent member of the axial magnetic bearing is displaced in a translational direction (horizontal direction).
In the above-described substrate rotating apparatus, radial magnetic bearings may be provided at one or a plural of axial position(s).
Preferably, the stator housing has a cooling zone in the vicinity of the processing chamber and further has a gas purge zone located across the cooling zone from the processing chamber.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings.